This invention relates to random noise generators and, in particular, to a random noise generator that utilizes the partial electrical breakdown along the internal surface of a housing to generate the random noise.
The prior art is filled with devices that generate white noise. White noise may be defined as random noise voltages of a reasonably uniform amplitude throughout a wide frequency spectrum. Generally, these devices utilize the breakdown or ionization characteristics of certain gases to generate the broad band noise. However, in recent years it has been found that utilizing semiconductor devices in their reverse or breakdown mode is also capable of generating broad band white noise. Typical patents which disclose broad band noise generators are as follows:
______________________________________ U.S. Pat. No. Issue Date Inventor ______________________________________ 2,637,003 April 28, 1953 Johnson, et al. 2,658,149 November 3, 1953 Gallagher, et al. 2,685,031 July 27, 1954 Johnson 2,842,712 July 8, 1958 Muldoon, et al. 2,855,514 October 7, 1958 Skolnik 3,199,048 August 3, 1965 O'Reilly, et al. ______________________________________
None of the prior art cited hereinbefore uses the surface of a glass housing for the electrical breakdown phenomena. Instead they utilize a gas or semiconductor disposed between anode and cathode elements. In the case of a semiconductor, the semiconductor is in intimate contact with the anode and cathode. Where an inert gas is used as the breakdown medium, the anode and cathode elements are spaced apart but in relatively close proximity to each other.
These sources of broad band noise find many applications in testing of receivers utilized in both commercial and military applications. These noise sources, generally, are of very low amplitude, e.g. thermal noise, which has a well established random quality. Often, the signal levels are so low that they require a high degree of amplification, which is costly and may degrade the original broad band noise characteristics of the original device.
The process of electrical breakdown may be generally divided into two types, one which is causal in nature, and one which is purely random or statistical. For example, the breakdown lag time of a gaseous spark gap may be described by a formative (causal) and a statistical (random) lag time component. In this case the initiation process occurs randomly in time, whereupon the formative process completes the breakdown. Since such breakdown usually involves relatively high voltage, high power noise signals may result. However, a disadvantage of this type of device lies in the need for the gap to recover to its original state, a factor which severely limits the repetition rate, and hence, the duty cycle.
Present observations show that partial discharges, that is, breakdown events which do not proceed fully to form a highly ionized conductive channel, develop along glass surfaces under the conditions of high electrical field stress. The high power pulses which occur are apparently the result of some cooperative process leading to aborted breakdowns which may occur at very high repetition rates (e.g. 10 MHz or more) depending upon the applied field, temperature, and light conditions.
The present invention utilizes a phenomenon heretofore not described in the prior art for the generation of random noise. Rather than relying on the ionization or breakdown of a gas or a semiconductor device, the instant invention relies on the partial discharge or breakdown events which occur along the inner surface of a rigid, electrically insulating, low leakage material such as glass, quartz or ceramic.